(1) Field of the Invention
The present invention relates to improvements in vibrators used to create seismic waves for seismic prospecting. More particularly, it relates to an improved seismic vibrator and to a method for improving the output of a seismic vibrator.
(2) Description of the Prior Art
In seismic prospecting, it is necessary to provide a source of energy for introducing waves into the earth formation to be explored. These waves propagate through the formation, are reflected in part by discontinuities in the formation, and subsequently are detected by geophones or other measuring devices at the earth's surface. The characteristics of the reflected waves are compared with the characteristics of the waves at their introduction into the formation. This comparison reveals valuable information about the structure of the formation and the probability of the presence of petroleum accumulations in the formation. In order to induce waves in the earth, it has become common to use mechanical devices known as seismic vibrators such as those illustrated in U.S. Pat. No. 3,929,206 (1975) to Bedenbender et al and U.S. Pat. No. 3,363,720 (1968) to Mifsud et al.
A typical seismic vibrator includes a baseplate in contact with or coupled to the earth, a holddown mass disposed above and connected to the baseplate so as to exert on the baseplate a downward or holddown force which tends to keep the baseplate in contact with the earth, a reaction mass connected to the baseplate to permit reciprocation of the reaction mass with respect to the baseplate, and a driver which reciprocates the reaction mass with respect to the baseplate in order to vibrate the baseplate at desired frequencies and amplitudes. The vibrations of the baseplate cause seismic waves to propagate through the earth. Usually elastic isolation springs, which may be referred to generally as compliant elements or compliant members, are interposed between the holddown mass and the baseplate to isolate the holddown mass from the vibrations of the baseplate, while at the same time maintaining the holddown force relatively constant. Frequently the driver includes a hydraulic piston or other reciprocating device which is responsive to an electric input signal.
In operation, an electrical input or sweep signal of know characteristics is impressed on the driver for the purpose of causing the baseplate to create seismic waves of similar characteristics in the earth. Typically, but not always, the sweep signal will be sinusoidal. The range of frequencies over which the baseplate is swept may be referred to as the sweep range and typically will be the range of frequencies from about 5 Hertz to about 100 Hertz. The vibrator may be swept up from the lower end of its sweep range to the higher end, or it may be swept down from the higher end of its sweep range to the lower end. The reflected seismic waves then are detected and compared with the sweep signal. Often a seismic vibrator will be mounted on a truck to carry it to desired locations and during operation all or part of the truck's weight is applied to the baseplate, so that the holddown mass referred to above will include all or part of the mass of the truck.
Most seismic vibrators of the type described above resonate with the earth when the baseplate is vibrating within a certain band of frequencies. This band of frequencies is quite narrow compared with the sweep range over which the vibrator operates, and the band may be referred to as the natural or resonant frequency of the vibrator-earth system, or simply the resonant frequency. This resonant frequency is a function of the impedance of the earth, the mass of the baseplate, the magnitude of the reaction mass, the stiffness of the isolation springs, and other factors. This resonant frequency does not necessarily occur within the sweep range, but frequently does and might be expected to occur for many vibrators presently in use between 15 Hertz and 25 Hertz. This invention is directed primarily to vibrators having a sweep range which includes the resonant frequency of the vibrator-earth system.
As a vibrator is swept through its sweep range, the magnitude of the displacement of the baseplate, and thus the amplitude of the wave generated in the earth formation, will increase as the vibration frequency approaches the resonant frequency and then will fall off as the frequency of vibration becomes less than or greater than the resonant frequency. This relative decline in the magnitude of the baseplate vibrations at frequencies different from the resonant frequency of the vibrator-earth system can be troublesome, because it increases the difficulty of recovering signals at these frequencies reflected from discontinuities in the underlying formation. This is particularly true at the high end of the sweep range, because the attenuation of compressional waves in the earth increases with the frequency of those waves.
Ross et al in U.S. Pat. No. 3,578,102 (1971) propose solving this problem by interposing a compliant element between the reaction mass and the baseplate and by continually adjusting the spring constant or stiffness of this compliant element in order to keep the vibrator at resonance with the earth throughout the generation of the seismic wave. This system appears to require rather precise calibration and maintenance and it may increase the difficulty of maintaining proper coupling between the baseplate and the earth. Wade in U.S. Pat. No. 3,106,982 (1963) proposes shifting the resonant frequency of the vibrator-earth system when the vibrator has a hydraulic driver by changing the volume of the driver's main cylinder. This solution does not permit changing the resonant frequency during a sweep and thus does not prevent the decrease in the amplitude of baseplate vibrations after resonant frequency for that sweep is reached. Crawford et al, while dealing with a different problem in U.S. Pat. No. 2,910,134 (1959), do recognize that changing the spring constant of the compliant element between the holddown mass and the baseplate will change the resonant frequency of the vibrator-earth system. However, like the Wade system, the Crawford system does not permit changing the resonant frequency during a sweep and does not prevent a decrease in the amplitude of the baseplate vibrations after the resonant frequency for that sweep is reached.